848
J. Nat. Prod. 1998, 61, 848-850
New Pyrrole Alkaloids from Solanum sodomaeum Khalid A. El Sayed,*,† Mark T. Hamann,†,‡ Hosney A. Abd El-Rahman,§ and Ahmed M. Zaghloul‡,§ The Department of Pharmacognosy and National Center for the Development of Natural Products and Research Institute of Pharmaceutical Sciences, University of Mississippi, University, Mississippi 38677, and Pharmacognosy Department, Faculty of Pharmacy, Mansoura University, Mansoura 35516, Egypt Received February 6, 1998
Two new pyrrole alkaloids, solsodomine A and B, were isolated from the fresh berries of Solanum sodomaeum L., collected from the Libyan desert. The structures of these compounds were established by 2D-NMR, including 15N NMR spectroscopy and chemical degradation. Solsodomine A (1) shows activity against Mycobacterium intracellulare. This is the first report of pyrrole alkaloids from the genus Solanum. Solanum is one of the largest genera in the plant kingdom. It includes about 1400 species widely distributed throughout the world.1,2 Solanum has been extensively studied for its steroidal alkaloids, nitrogencontaining compounds, and isoprenoids.3 Solanum alkaloids exhibit an interesting range of bioactivity. Solapartine, solapalmitine, and solapalmitenine show tumor inhibitory activity against human nasopharynx carcinoma (KB).4 Solamargine possesses potent cytotoxic activity to human hepatocyte (Hep3B) and normal skin fibroblasts.5 Other Solanum alkaloids show significant antifungal and antihepatotoxic activities.6,7 Solanum sodomaeum L. (Solanaceae) is an annual herb, wildly distributed in the Libyan desert.8 A topical formulation containing purified glycoalkaloids from S. sodomaeum L. has proven effective in the treatment of malignant human skin tumors; basal cell carcinomas, squamous cell carcinomas, and the benign tumors; keratoses, and keratoacanthomas.9 Another study of the mixture of glycoalkaloids extracted from the title plant indicated its antineoplastic activity against Sarcoma 180 in mice.10 This paper represents the first study of the less polar nonsteroidal alkaloids from S. sodomaeum and the first report for pyrrole alkaloids from the genus Solanum. The alkaloid-containing extract of fresh berries of S. sodomaeum L. was fractionated into the less polar (CHCl3) and the more polar (EtOAc) fractions. Unlike the EtOAc fraction, the CHCl3 fraction did not give positive Lieberman-Burchard, Salkowski, or Molish’s tests, indicating the absence of steroidal glycosides.11 TLC screening of the CHCl3 fraction indicated the presence of alkaloids by a positive Dragendorff’s test.11 Fractionation of the CHCl3 extract on silica gel 60 afforded two new pyrrole alkaloids, named solsodomine A (1) and B (2). The FTIR spectra of 1 showed absorptions at 3465 and 1657 cm-1, suggesting the presence of NH and N-formyl functionalities. The CIMS spectra of 1 displayed a molecular ion peak at m/z 204, consis* To whom correspondence should be addressed. Tel: (601) 2327184. Fax: (601) 232-7026. E-mail:
[email protected]. † Department of Pharmacognosy, University of Mississippi. ‡ National Center for the Development of Natural Products and Research Institute of Pharmaceutical Sciences, University of Mississippi. § Pharmacognosy Department, Mansoura University.
S0163-3864(98)00042-1 CCC: $15.00
tent with the molecular formula C10H12ON4 and the presence of seven degrees of unsaturation. The proton singlet at δ 9.40 is correlated to a methine carbon resonating at δ 179.1, indicating the presence of an N-formyl group (Table 1). The downfield methylene triplet (δ 4.44) is correlated by the HMQC experiment to the nitrogenated methylene carbon at δ 48.6 (C-7). The C-7 methylene group displayed COSY coupling to another methylene triplet (δ 2.93), which is correlated in the HMQC spectrum to the methylene carbon at δ 28.6 (C-8). The presence of the pyrrole functionality was indicated from the two COSY-coupled doublet of doublets resonating at δ 6.05 and 6.87, correlating to the two methine carbons at δ 109.3 and 125.3 (C-4 and C-5, respectively). Both protons (δ 6.05 and 6.87) show longrange COSY couplings to a broad proton singlet resonating at δ 6.75, correlated to the methine carbon at δ 131.9 (C-2). The imidazole moiety was suggested from the two long-range COSY-coupled singlets resonating at δ 7.47 and 6.57, which are correlated to the methine carbons absorbing at δ 134.2 and 116.6 (C-11 and C-13 respectively). Analysis of the HMBC data of 1 further supported the proposed structure (Figure 1). The 3JHMBC correlations of H-14 with C-3 and C-7 supported its location on N-6. The 3J-HMBC correlations of H-2 with C-4 and C-5 further supported the pyrrole ring entity. The same proved true for the imidazole ring with 3J-HMBC correlations between H-11, C-9, and C-13. Additional confirmation was obtained from the analysis of 1H-15N HMBC data (Figure 1, Table 1). The downfield nitrogen resonating at δ 225.2 (N-10) show 3J-HMBC correlations to H-8 and H-13, in addition to a 2J-HMBC correlation to H-11. The nitrogen resonating at δ 190.8 (N-12) shows 2J-HMBC correlations to H-11 and H-13. The nitrogen (N-1) resonating at δ
© 1998 American Chemical Society and American Society of Pharmacognosy Published on Web 05/28/1998
Notes Table 1. 2a
Journal of Natural Products, 1998, Vol. 61, No. 6 13C
and 1H NMR Spectral Data of Compounds 1 and 1
position
δC
1 2 3 4
131.9, d 130.4, s 109.3, d
5
125.3, d
6 7 8 9 10 11 12 13 N6-formyl
δH 5.86, br s 6.75, br s
2 δN
δC
165.8 132.9, d 132.8, s 109.8, d
6.05 dd (4.0, 2.5) 6.87 dd (4.0, 2.4)
125.7, d
δH 5.87, br s 7.03, br s 6.22 dd (4.1, 2.5) 6.96 dd (4.1, 2.4)
165.6 48.6, t 28.6, t 133.5, s
4.44 t (7.2) 2.93 t (7.3)
134.2, d
5.86, br s 7.47, s
116.6, d 179.1, d
6.57, s 9.40, s
51.1, t 50.3, t 225.2
31.8, q
4.42 t (7.1) 3.84 t (7.2) u 3.41, s
179.8, d
9.42, s
190.8
a
In CDCl3, at 400 MHz for 1H, 100 MHz for 13C, and 50 MHz for 15N. Carbon multiplicities were determined by the DEPT 135° experiment; s ) quaternary, d ) methine, t ) methylene, q ) methyl carbons, u ) unobserved; coupling constants (J) are in Hz.
Figure 1. Important 1H-13C (solid lines) and 1H-15N (dashed lines) HMBC correlations of 1.
165.8 shows 2J-HMBC correlations to H-2 and H-5, as well as a 3J-HMBC correlation to H-4. Similarly, the nitrogen (N-6) absorbing at δ 165.6 showed 2J-HMBC correlations to H-7 and the downfield N-formyl proton singlet, in addition to 3J-HMBC correlations to H-2, H-4, and H-8. The downfield shifting of the aliphatic N-6 is attributed to the deshielding effect of the N-formyl moiety. Acid hydrolysis of 1 afforded 3 which has similar 1H NMR spectral data except for the absence of the N-formyl signal. The CIMS spectra of solsodomine B (2) displayed a molecular ion peak at m/z 168 [M + H]+, consistent with the molecular formula C8H13ON3 and the presence of four degrees of unsaturation. The FTIR and 1H and 13C NMR spectra (Table 1) of 2 showed distinct similarity to those of 1 except for the lack of the imidazole moiety and the emergence of a new N-methyl group. The proton singlet resonating at δ 3.41 correlated to the methyl carbon at δ 31.8 (C-10). The downfield shift of the methylene proton triplet resonating at δ 3.84 and correlated to the methylene carbon at δ 50.3 is attributed to the nitrogenated C-8. Solsodomine A (1) shows moderate activity against Mycobacterium intracellulare with an MIC of 10 µg/mL. Compounds 1 and 2 did not show in vitro antimalarial activity against Plasmodium falciparum W2 and D6 clones, cytotoxic activity to Vero cells, or antifungal activity against Cryptococcus neoformans and Candida albicans. This is the first report for pyrrole alkaloids in the genus Solanum; however, they have been previously
849
reported in the family Solanaceae from Nicotiana bigelovii and Nicotiana setchellii .2 Experimental Section General Experimental Procedures. Melting points were determined on a Thomas-Hoover capillary melting point apparatus and are uncorrected. UV spectra were acquired on a Perkin-Elmer Lambda 3B UV/vis spectrophotometer. The IR spectra were recorded on an ATI Mattson Genesis Series FTIR spectrophotometer. The 1H and 13C NMR spectra were recorded in CDCl on a 3 Bruker DRX-400 spectrometer. The 15N NMR was recorded on a Bruker DRX-500 spectrometer. The LRCIMS spectra were obtained at the University of Kansas on a Fisons/VG Autospec Q mass spectrometer. TLC analysis was carried out on precoated silica gel G254 (Merck) or aluminum oxide ALOX-100 UV254 (MachereyNagel) plates. For column chromatography, Baker silica gel 60, 40 µm was used. Plant Material. Fresh S. sodomaeum L. berries were collected near Tripoli during spring 1994. The plant was identified by Dr. A. El-Gadi, Department of Botany, Al Faateh University, Tripoli, Libya. A voucher specimen (94Trip-3) was deposited at Pharmacognosy Department, Mansoura University, Egypt. Extraction and Isolations. Fresh S. sodomaeum L. berries (4.5 kg) were minced, and the formed paste was extracted twice each with 2 L of 95% hot ethanol and concentrated under reduced pressure. The semisolid ethanolic extract (750 g) was treated with 1 L of 5% aqueous tartaric acid and filtered. The filtrate was extracted twice, each with 1 L of CHCl3. The acidsoluble fraction was alkalinized to pH 9, using concentrated NH4OH, and then extracted with CHCl3 (1 L × 3) followed by EtOAc (1 L × 3). The chloroform fraction was subjected to repeated column chromatography on silica gel 60 using n-hexane and gradient elution with EtOAc. The alkaloid-containing fractions were further subjected to preparative TLC on Al2O3, using cyclohexanes-EtOAc (2:1) to afford compounds 1 (85.5 mg, Rf 0.45) and 2 (2.2 mg, Rf 0.71). Solsodomine A (1): yellow needles from EtOH; mp 118-119 °C; UV λmax (log ) (MeOH) 280 (3.08), 320 (3.06), 348 (2.93) nm; IR υmax (CHCl3) 3465, (NH), 30062931, 1657 (N-CHdO) cm-1; 1H and 13C NMR data, see Table 1; LR-CIMS m/z (fragment) 204.0 (M+, 15) (calcd for C10H12ON4, 204.10), 190.0 (95), 176 (25), 162 (65), 94 (55), 81 (30), 69 (20). Acid Hydrolysis of 1. Forty mg of 1 was dissolved in 1 mL of EtOH, and 5 mL of 6 N HCl was added. The reaction mixture was heated for 120 min at 80 °C. The pH was adjusted to 7, using dilute NH4OH, and then the reaction mixture was extracted with CHCl3 (15 mL × 2). The CHCl3 solution was evaporated and subjected to preparative TLC on silica gel G254 using CHCl3MeOH (9:1) to afford compound 3 (3.8 mg, Rf 0.32). Solsodomine B (2): colorless needles from EtOH; mp 112-113 °C; UV λmax (log) (MeOH) 271 (3.10), 317 (3.01), 342 (2.03) nm; IR υmax (CHCl3) 3455, (NH), 29952930, 1655 (N-CHdO) cm-1; 1H and 13C NMR data, see Table 1; LR-CIMS m/z (fragment) 168.0 [(M + H), 10]+ (calcd for C8H14ON3, 168.11), 124.0 (100), 79 (15), 43 (50). Compound 3: yellow needles from EtOH; mp 122123 °C; UV λmax (log ) (MeOH) 266 (2.99), 319 (3.11),
850 Journal of Natural Products, 1998, Vol. 61, No. 6
341 (2.71) nm; IR υmax (CHCl3) 3455, (NH), 3010-2950 cm-1; 1H NMR data (400 MHz, CDCl3) δ 6.42 (1H, brs, H-2), 6.10 (1H, dd (4.0, 2.1), H-4), 6.53 (1H, dd (4.0, 2.2), H-5), 3.97 (2H, t (7.1), H7), 2.96 (2H, t (7.1), H-8), 7.43 (1H, s, H-11), 6.56 (1H, s, H-13). Acknowledgment. The authors are grateful to Dr. A. El-Gadi, Department of Botany, Al Faateh University, Tripoli, Libya, for the taxonomic identification of the title plant and D. Charles Dunbar for running the 15N NMR spectra. References and Notes (1) Hegnauer, R. In Chemotaxonomie Der Pflanzen; Birkhauser Verlag: Basel, Stuttgart, 1973; Vol. 6, pp 405-430. (2) Hegnauer, R. In Chemotaxonomie Der Pflanzen; Birkhauser Verlag: Basel, Boston, Berlin, 1990; Vol. 9, pp 569-584.
Notes (3) Evans, W. C.; Somanabandhu, A. Phytochemistry 1980, 19, 2351-2356. (4) Kupchan, S. M.; Davis, A. P.; Barboutis, S. J.; Schnoes, H. K.; Burlingame, A. L. J. Org. Chem. 1969, 34, 3888-3893. (5) Hsu, S. H.; Tsai, T. R.; Lin, C. N.; Yen, M. H.; Kuo, K. W. Biochem. Biophys. Res. Commun. 1996, 229, 1-5. (6) Kusano, G.; Takahashi, A.; Sugiyama, K.; Nozoe, S. Chem. Pharm. Bull. 1987, 35, 4862-4867. (7) Lin, C. N.; Gan, K. H. Planta Med. 1989, 55, 48-50. (8) Jafri, S. M. H.; El-Gadi, A. In Flora of Libya; Al-Faateh University: Tripoli, 1978; Vol. 62, pp 10-12. (9) Cham, B. E.; Meares, H. M. Cancer Lett. 1987, 36, 111-118. (10) Cham, B. E.; Gilliver, M.; Wilson, L. Planta Med. 1987, 53, 3436. (11) Harborne, J. B. In Phytochemical Methods A Guide to Modern Techniques of Plant Analysis, 2nd ed.; Chapman and Hall: London, New York, 1984; pp 197-198.
NP980042P
